No abstract

Non-microbial methane (NM-CH4), emissions from soil might play a significant role in carbon cycling and global climate change. However, the production mechanisms and emission potential of soil NM-CH4 from tropical rainforest remain highly uncertain. In order to explore the laws and characteristics of NM-CH4 emission from tropical rainforest soils. Incubation experiments at different environmental conditions (temperatures, soil water contents, hydrogen peroxide) and for soils with different soil organic carbon (SOC) contents were conducted to investigate the NM-CH4 emission characteristics and its influence factors of soils (0-10cm) that collected from a tropical rainforest in Hainan, China. Incubation results illustrated that soil NM-CH4 release showed a linear increase with the incubation time in the first 24 hours at 70 °C, whereas the logarithmic curve increase was found in 192 h incubation. Soil NM-CH4 emission rates under aerobic condition were significantly higher than that of under anaerobic condition at first 24 h incubation. The increasing of temperature, suitable soil water contents (0–100%), and hydrogen peroxide significantly promoted soil NM-CH4 emission rates at the first 24 h incubation. However, excessive soil water contents (200%) inhibited soil NM-CH4 emissions. According to the curve simulated from the NM-CH4 emission rates and incubation time at 70 °C of aerobic condition, soil would no longer release NM-CH4 after 229 h incubation. The NM-CH4 emissions were positively corelated with SOC contents, and the average soil NM-CH4 emission potential was about 6.91 ug per gram organic carbon in the tropical mountain rainforest. This study revealed that soils in the tropical rainforest could produce NM-CH4 under certain environment conditions and it supported production mechanisms of thermal degradation and reactive oxygen species oxidation. Those results could provide a basic data for understanding the soil NM-CH4 production mechanisms and its potential in the tropical rainforest.

Studies on land degradation and development (LDD) and the underlying factors can help land restoration. In this study, the LDD of Hainan Island from 2000 to 2018 were measured by change vector analysis using the normalised difference vegetation index and net primary productivity (NPP). The underlying factors of LDD were selected from seven potential factors (nature factors: mean annual temperature, mean annual drought intensity, human disturbances: gross domestic product (GDP), population, population urbanisation, accumulated afforestation area, construction area) based on structural equation modelling. The results showed that 1) the degree of land-use changes into water area and construction land were higher than for the other land-use types; 2) Compared to 2000, the most serious degradation occurred in 2015, which accounting for 68.98% of the total land degradation as well as more than three-quarters of the land degradation in all surveyed years was driven by NPP; 3) The proportion of land degradation (PLD) was negatively correlated with the mean value of the land degradation and development (MLDD) in the period of five years, 2000-2018 (P<0.01); 4) The key influencing factors of PLD were temperature, accumulated afforestation area, population urbanisation, in 2000–2005, 2005–2010 and 2010–2018, respectively; 5) The key influencing factors of MLDD were temperature, GDP, population urbanisation, in 2000–2005, in 2005–2010 and in 2010–2018, respectively. Therefore, to minimise land degradation it is necessary to limit the speed of population urbanisation occurs and to improve the island environment’s adaptability to extreme climates.